This invention relates generally to engine preheaters, and more particularly to an improved engine preheater for heating engine coolant to warm the engine when it is not running.
That engines are difficult to start and subject to excessive wear when operated in cold, northern areas has been common knowledge for many years. Various kinds of engine preheaters that heat engine coolant and/or engine oil have been used to ameliorate these problems. Heretofore, while preheaters have heated engine coolant while the engine is off thereby increasing the reliability and lifespan of the engine, the preheaters have not always been reliable. Problems have included thermostat "burn-out" due to excessive cycling of the thermostat and "dry fires" in the preheater due to no-flow or low-flow conditions when a coolant hose is either partly or completely clogged or pinched shut, or when the flow of coolent is reversed by operation of the engine, thereby closing preheater valve(s) and stopping flow.
The thermostat may be "burned-out" by its excessive cycling (thermostat "chatter"), which is sometimes caused by placing the temperature sensor for the thermostat too close to the heating element controlled by the thermostat, in which case the sensor detects rapid changes in temperature after the heating element starts to heat up or cool down. The farther the temperature sensor is from the heating element, therefore, the less frequently the thermostat is likely to cycle.
"Dry fires", on the other hand, are caused by the preheater being low or short of coolant because of low coolant level in the coolant system, or no-flow conditions due to preheater valve(s) closed by back flow through the preheater during engine operation. "Dry fires" may be prevented by placing the temperature sensor and the heating element close to one another in locations where nothing is likely to interfere with the sensor's detection of excessive heat. If the sensor and heating element are close to each other, dry fires are prevented by the ability of the sensor to detect the increased temperature so that the thermostat may turn the heating element off.
Another approach to preventing "dry fires", at least when caused by backflow while the engine is running, has been to provide a pressure switch to deactivate the preheater when the engine is running. However, the addition of a pressure switch increases the cost and complexity of the preheater.
Typically, preheaters are designed to prevent thermostat "burn-out" by placing the thermostat and temperature sensor outside of the preheater along the coolant line carrying coolant to be heated. This approach successfully reduces the problem of excessive cycling of the thermostat while ignoring the problem of dry fires in the preheater. In addition, wiring associated with a sensor and thermostat placed separately along the coolant line may become entangled with other wires and hoses, thereby making installation of the preheater difficult and complicated.